= Impedance head
An <impedance head> combines a force transducer and an <accelerometer> in one housing for a driving-point vibration test. It measures applied <force> and acceleration at nearly the same location and along the intended excitation direction. The raw channels yield <mechanical accelerance> or its reciprocal <mechanical dynamic mass>; deriving <mechanical driving-point mobility> or <mechanical driving-point impedance> additionally requires the acceleration-to-velocity conversion.<Impedance Heads: Types 8000 and 8001 by Brüel and Kjær>[Printed pp.1–2, construction, separate output sockets and point-mobility/point-impedance footnote]{ref}
In the manufacturer's Types8000/8001 example, a piezoelectric acceleration sensor sits above a force gauge in a stiff housing, with separate electrical outputs. The arrangement aims to preserve the phase relation needed for a complex response ratio. This solves a practical apparatus problem: a separate exciter and response sensor can otherwise occupy different points, particularly on a small structure.
An <impedance-head mass correction> accounts for moving material between the force-sensing plane and the specimen. In a rigid lumped approximation, choose a common positive axis and let that material have effective mass $m_h$ and acceleration <complex amplitude> $A$. Then <Newton's second law> gives
$$
F_{\rm gauge}=F_{\rm specimen}+m_hA.
$$
For an original in-phase example, let $m_h=0.020\,\mathrm{kg}$, $A=10\,\mathrm{m/s^2}$ and gauge force $1\,\mathrm N$. Specimen force is $0.80\,\mathrm N$, so specimen accelerance is $12.5\,\mathrm{kg^{-1}}$ rather than the uncorrected value10. With general signals the correction is a complex subtraction, not subtraction of unsigned amplitudes.
This force-plane correction does not undo every effect of attaching the instrument: added mass and mounting stiffness can change the specimen's actual vibration. The test must also state mounting, calibration, usable frequency range and the motion of any second terminal. Manufacturer guidance distinguishes measurements requiring mass compensation and cases where separate force/acceleration sensors are preferable.<Impedance Heads: Types 8000 and 8001 by Brüel and Kjær>[Printed p.2, measurements on light/heavy structures, mass compensation and Figure2]{ref}
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